Self-regulating heating device



Dec. 3, 1968 c. D. FLANAGAN ETAL SELF-REGULATING HEATING DEVICE Filed Jan. [5, 1966 INVENTORS CHARLES D. FLANA GAN,

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United States Patent 3,414,706 SELF-REGULATING HEATING DEVICE Charles D. Flanagan, Attleboro, Mass., and Leo Marcoux,

Pawtucket, R.I., assignors to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Jan. 3, 1966, Ser. No. 518,277 13 Claims. (Cl. 219-210) ABSTRACT OF THE DISCLOSURE Method and means for providing a relatively constant ambient temperature for temperature sensitive devices in which a plurality of heater elements composed of material having a positive-temperature coefficient of resistance (hereinafter referred to as PTC) are placed adjacent one another in ascending order of PTC anomaly temperatures resulting in a temperature control gain in the direction of the higher PTC anomaly temperatures. One embodiment shows a first set of stacked annular PTC discs placed within a second set of stacked annular PTC discs and electrically connected in parallel thereto. The constant ambient temperature is thus provided in the center of the inner set. More than two sets of discs may be employed as well as configurations other than annular.

This invention relates to heating elements, and particularly to ovens used to provide a constant temperature for temperature sensitive devices located therein. There are many electronic components which must be contained in a constant ambient for effective operation in certain applications. Examples of such components are crystals, diodes, transistors and so on.

It is known to provide ovens as described above which utilize a heater and a thermostat. The thermostat keeps the inside oven temperature within a certain range by turning on and off the heater current by use of movable contacts. This type of oven has certain inherent disadvantages, viz., the temperature varies as the result of the characteristics of the thermostatfrom a maximum to a minimum back to a maximum and so on. Also, since there is a mechanical movement, the longevity of the device is limited.

Another approach has been what is commonly known as proportional control whereby relatively complex electrical circuits serve to limit the power input to the heater to equal the-heat loss from the oven. This is done, for example, by providing a bridge containing a temperature-sensing device which is used to balance a circuit containing the heater. This type of control eliminates the on/off moving contacts and therefore provides more precise temperature control with no overshoot or thermal cycling, more constant power requirements and no noise due to mechanical operation although many devices of this description do emit electrical noise. Also, the device is relatively complex and expensive. In copending and coassigned applications Ser. No. 435,166, filed Feb. 25, 1965, and Ser. No. 435,165, filed Feb. 25, 19-65, devices were disclosed and claimed which provide such a constant ambient utilizing self-regulating heating elements. It is an object of this invention to provide an improved selfregulating heating device, especially of the type contained in the first mentioned application.

It is an object of the invention to provide an oven which is simple, highly reliable, long lasting, mechanically and electrically silent in operation and displays a closely controlled, relatively constant oven temperature.

It is another object of the invention to provide a heating element which has a self-regulated temperature.

It is a further object to provide an oven which is 3,414,706 Patented Dec. 3, 1968 "ice characterized by being of a self-regulating temperature nature.

It is another object of the invention to provide an improved self-regulating oven which will maintain a relatively constant inside temperature for temperature sensitive devices contained therein regardless of change in heat demand which is simple, reliable, long lasting, silent operating and has no moving parts.

The invention accordingly comprises the elements and combinations of elements, steps and sequence of steps, features of construction and manipulation and arrangements of parts, all of which will be exemplified in the structures and methods hereinafter described, and the scope of the application of which will be indicated in the appended claims.

In the accompanying drawings in which one of the various possible embodiments of the invention is illustrated:

FIG. 1 is a vertical partial cross sectional view through one embodiment of the invention; and

FIG. 2 is a schematic wiring diagram of the oven of FIG. 1.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

Dimensions of certain of the parts as shown in the drawings may have been modified and/or exaggerated for the purposes of clarity of illustration.

Referring now to the drawings, numeral 10 relates to the device or oven generally. It comprises a discshaped base 12 which may be formed of a conventional moldable phenolic resin or other suitable electrically insulating material and which mounts a heater assembly 14 composed of two sets of generally annular discs, 16 and 18 which will be further described infra. Heater assembly 14 is formed with a cavity 20 therein in which temperature sensitive components 9 are mounted. Although it is not essential, a thermally conductive liner 22, e.g., aluminum, may be placed in cavity 20 to even out any temperature gradients which might exist in an axial direction. Electrical insulation 24 and 26 is used to prevent any short circuiting of the heater current. Quick disconnect pins 28 and 29 are mounted in a conventional potting compound 30 which is of an electrical as well as a heat insulating material conventional in the art. Insulation 30 will keep heat losses of the oven to a minimum. Pins 28 and 29 protrude from potting material 30 as shown in FIG. 1 and are received in mating clips 31 and 32 respectively which in turn are contained in bores 33 formed in base 12. A conventional mounting assembly 36 is attached to the bottom of base 12 which includes shank 38 formed with key 40 which serves to properly orientate the oven for insertion in a receiving means (not shown). The mounting assembly 36 mounts pins 18 as shown, the number of which is a matter of choice. The heater, self-regulator assembly 14, is composed of sets 16 and 18 of discs made of material having a steep positive sloped, resistivity-temperature curve at temperatures above the anomaly point (hereinafter referred to as PTC material). A layer of silver or other electrically conductive material is attached by conventional means to the two faces of each of the discs, the layers shown in FIG. 1 number from 41-52.

Set 16 is composed of annular discs 54 sandwiched between discs 55 and 56. Set 18 is composed of annular discs 58 sandwiched between discs 59 and 60. It will be seen that set 16 is telescopically received within the coaxial recess formed within set 18. Although it is not essential a thermal insulation layer 62 is shown separating the two sets from another (this provides a thermal gradient which will be explained in greater detail infra).

The even numbered layers 42-48 are electrically connected to conductor 64 by conductors 65. Conductor 64 is electrically connected to pin 29. Layers 50 and 52'are electrically connected to clip 32 by conductor 66 and 67 respectively. Odd numbered layers 41-49 are connected to conductor 68 by conductors 69. Conductor 68 is electrically connected to quick disconnect pin 28. Layer 51 is electrically connected to clip 31 by conductor 70.

It will be seen that this provides two stacks of discs, one within the other, connected in parallel. It will also be obvious that the particular number of discs employed is a matter of choice.

Bottom discs 56 and 60 are provided with bores 72 to provide space for leads 73, 74 and 76, 77 of sockets 75, 78 respectively.

FIG. 2 shows a schematic wiring diagram indicating the internal electrical connections of pins 1-8. Conductors 80 and 81 connect clips 31 and 32 and hence the PTC discs to pins 3 and 1. Leads 73 and 74 are connected to pins 4 and 6 respectively and leads 76 and 77 are connected to pins 2 and 8 respectively. Pin is grounded and pin 7 is not used. Metal cap 86 encloses oven 10 and is attached to base 12 by any conventional means, such as screws 87 shown.

Heating and regulating current (AC or DC) is applied through the PTC sets 14 and 16 from pin 1 to conductors 81, clip 32, pin 29, conductors 64-67, even numbered layers 42-52, PTC elements 54-56 and, 58-60, odd numbered layers 41-51, conductors 69, 70 through conductor 68, pin 28, clip 31, conductor 80 and pin 3. The PTC discs act as a heater and also as their own temperature regulator. The current passing through the PTC material causes heat to be generated thereby heating up the oven cavity 11. Once the oven is warmed up, very little temperature variation occurs within cavity 20 regardless of ambient temperature fluctuations outside the oven, i.e., changes in heat demand, or appreciable fluctuations in the applied voltage. A relatively constant ambient is provided in the oven for the components contained therein and this is achieved without moving parts and without any elaborate, comparatively complex circuitry. The relatively low heat generation of components within the oven has been found to have a negligible effect on the inside temperature of the oven.

At temperatures above the anomaly very little heating occurs since resistivity of the PTC material increases much more rapidly than temperature at temperatures above the anomaly. Therefore, heat generation, which is inversely proportional to resistivity, drops off drastically above the anomaly. If heat demand is increased tending to lower the temperature there is a relatively great drop in resistance with a concomitant increase in current (E=IR) and likewise generated heat (P=l R) until the material again attains a balance with heat dissipation equal to heat generated. Because of the steep slope of the PTC material very little temperature fluctuation occurs during such changes in resistance. The inner surface temperature variation of set 18 therefore, will be slight thereby providing a relatively stable temperature for the outer surface temperatuer of set 16 which in turn results in a very stable or constant temperature for the inside surface of set 16 and hence cavity 20.

'For the successful operation of the oven within the purview of the invention, the self-regulating heating element assembly 18 must be constructed of material having as a characteristic :1 large positive temperature coefficient of resistance (PTC); that is, material in which the percent change in resistance per degree change in temperature the so called break-point range is very large, for example from to 150% per degree centigrade. This break-point range occurs near the Curie point of the material in the case of ceramic like material such as lanthanum doped barium titanate Ba La TiO disclosed in the above-mentioned copending applications and in the case of materials disclosed in copending and coas- 4 signed application Ser. No. 472,108, filed July 15, 1965, at the anomaly point. For a further explanation of the self-regulating characteristic of the PTC material reference may be had to these applications.

Closer temperature control is achieved in this device by using two sets of discs 14, 16 one placed within the other. The PTC anomaly temperature of set 14 has a lower value than that of the inner set 16. We have found that in a case of a single set of discs a control gain of about 30 can be had (a 30 variation in outside ambient temperature will result in a 1 variation in internal oven temperature) but by use of a double set of discs one Within another a control gain of approximately 900 is achieved (a 300 variation in outside ambient temperature resulting in a /3 inside variation).

Effective temperature control is realized through the use of a controlled temperature gradient existing between the two sets of PTC discs 14, 16. By using material having two different anomaly temperatures such a controlled gradient is effected. This gradient can be further maintained by using insulating layer 62 between sets 14, 16 if desired.

The outside surface of set 18 is subjected to a relatively wide fluctuation in temperature (dependent upon outside ambient conditions). The inner surface of set 18 has a narrower range of fluctuations or temperature gradient due to the heating characteristics of the PTC material. This is referred to as control gain. The temperature range of the inner surface of set 18 is effectively the samefor the outer surface of set 16. Again a narrower temperature gradient results at the inner surface of set 16 and hence cavity 20 due to the same control gain char acteristics of the PTC material.

It will be obvious that a greater control gain can be achieved by using a greater number of PTC sets, each set having an ascending anomaly temperature, in the direction of the control gain (cavity 20).

Instead of defining a cavity with the PTC layers it is within the purview of the invention to effect the control gain by employing a plurality of layers of any geometry, e.g., sheets, each layer having a higher anomaly temperature in the direction of the control gain. Thermal insulation can be used to minimize heat loss in the environs of the area to be controlled.

The PTC material is chosenso that the oven operates inherently in the range which includes the anomaly point. One such material is lanthanum doped barium titanate the preparation of which is described in application Ser. No. 435,166 and Ser. No. 435,165. The anomaly point is controlled by the kind and amount of doping employed. Another such material is the carbon black filled crosslinked polyethylene in application Ser. No. 472,108. The anomaly temperature in the latter material may be modified by the amount of carbon present, the amount of closs linking, etc. to provide the different operating characteristics used in the two sets 14, 16.

Any material which displays a relatively steep, positive sloped, resistivity-temperature curve can be used as the heat generating-self regulating element in accordance with the present invention.

Instead of employing conductive layers 41-52 metal tabs may be inserted between the PTC discs to provide the electrical connections therefor. It should be noted that separate voltage sources could be supplied to the several discs of equal or varying magnitude to provide a desired axial temperature gradient if desired. 1

In view of the above it will be seen that the several objects of the invention are achieved and other advantageous results attained.

-It is to be understood that the invention is not limited in its application to the details of construction, methods and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to tbe understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

As many changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

We claim:

1. A self-regulating heating device comprising:

(a) a first steep-sloped PTC element having at least two spaced portions and a cavity defined therein;

(b) a second steep-sloped PTC element having at least two spaced portions and a cavity defined therein received in the cavity of said first PTC element; and

(c) means to apply voltage across the two spaced portions of said first and second PTC elements whereby the resulting current will cause the element to heat up and an equilibrium will be reached so that the temperature of the surface defining the cavity of said second PTC element as well as the temperature within the cavity will be approximately constant regardless of change in heat demand.

2. A self-regulating heating device according to claim 1 in which the PTC elements are hollow cylinders and the means to apply voltage across the spaced portions includes an electrically conductive layer electrically connected to each spaced portion.

3. A self-regulating heating device according to claim 1 in which the PTC material is lanthanum doped barium titanate.

4. A self-regulating heating device according to claim '3 in which the lanthanum doped barium titanate is .99'l .003 3- 5. A self-regulating heating device according to claim 1 in which the anomaly temperature of the first PTC element is lower than the anomaly temperature of the second PTC element.

6. A self-regulating heating device according to claim 1 in which the PTC element includes a plurality of annular discs made of PTC material stacked one on top of the other to form hollow cylinders, an electrically conductive layer electrically connected to each face of the discs and the discs connected in parallel.

7. An oven for maintaining a relatively constant ambient temperature for temperature sensitive devices regardless of changes in heat demand comprising:

(a) a first set of generally annular discs formed of PTC material stacked one on top of another to form a cylinder with a cavity therein;

(b) a second set of generally annular discs formed of PTC material stacked one on top of another to form a cylinder with a cavity therein, said second set of discs telescopically received and'closely fitting in the cavity of said first set of discs;

(c) an electrically conductive layer attached to each face of the discs;

(d) two disconnect pins;

(e) leads connecting the pins to the layers so that the discs are connected in parallel;

(f) electrically insulating potting compound encasing the leads, part of each pin and all but one end of the sets of PTC discs;

(g) electrical connecting means mounted in the :base

to receive and removably hold the pins;

(h) a base of electrical insulation material which supports the encased PTC discs;

(i) a socket mounted on the base for mounting a temperature sensitive device and adapted to be received within the cavity of said second set of discs;

(j) oven mounting means attached to the base and providing electrical connections for the socket and pins through the pin receiving and holding means; and

(k) a cover enclosing the sets of PTC discs and releasably attached to the base.

8. An oven according to claim 7 further including:

(1) thermal and electrical insulation separating the said sets of PTC discs; and

(m) :a cylindrical thermally conductive liner telescopically received within the cavity of said second set of discs; and

(n) electrical insulation separating the liner from the said second set of discs.

9. An oven according to claim 7 in which the anomaly temperature of the first set of PTC discs is lower than the anomaly temperature of the second set of PTC discs.

10. An oven according to claim 7 in which the PTC discs are composed of 'Ba 997La- 003TiO3.

-11. An oven according to claim 1 in which the PTC elements are discs composed of carbon black filled, cross-linked polyethylene.

12. The method of providing a relatively constant ambient temperature for temperature sensitive devices comprising the steps of defining a temperature controlled a ea; taking a plurality of PTC material layers having different anomaly temperatures; placing the layers adjacent one another in ascending order of PTC anomaly temperatures with the layer having the highest anomaly References Cited UNITED STATES PATENTS 2,892,988 6/1959 Schusterius 338-9 2,953,759 9/1960 Lehovec 3388 3,038,056 6/ 1962 Wolfskill 2192l0 3,295,087 12/1966 Landis et a1. 338-28 RICHARD (M. WOOD, Primary Examiner.

C. L. ALBRITION, Assistant Examiner. 

